Constant velocity joints are a special class of universal joint in which the angular velocity of the output and input shafts match identically, irrespective of the angle of the joint. Due to this characteristic of uniform velocity transfer, CV joints find significant application in front wheel drive automobiles, and are usually applied two per drive axle and thus four per vehicle.
A CV joint generally comprises an inner race, an outer race, a ball cage and coupling balls. Both the inner and outer races have ball tracks for accepting the coupling balls which transfer torque from one race to the other. When the axle is rotated with the CV joint flexed to accommodate a given angle between input and output shafts, the coupling balls travel back and forth in the ball tracks.
Because CV joints on drive axles are used to transfer torque from the engine to the driving wheels, they are often operated under high stress. The joints are lubricated in manufacture, and a flexible boot installed to seal the joint from dust and moisture. The boots can crack or tear, which will allow the entry of moisture or dirt, the deterioration of the lubricant, and the accelerated wear of the joint or its components. Even in cases where the boot is not damaged, the joint can wear over time. Often the wear exhibits itself as shallow grooves formed in the tracks.
A defective joint will usually be manifest by a rhythmic clicking sound when the joint is flexed and under load, or in more extreme cases by significant vibration. The clicking is especially predominant when the vehicle is under load, such as when accelerating around a turn. Indeed, oftentimes a defective joint will not exhibit the telltale clicking, even with the joint flexed, if the joint is not operated under significant load.
The difficulty in isolating driveaxle problems from other types of noise and vibration will be apparent. Since the clicking and sometimes the vibration are only exhibited under load while in a turn, and since there are numerous other components of the automobile which can click and vibrate under the same conditions, it takes a degree of skill and experience to pinpoint a problem as a defective driveaxle.
Even though the technology for manufacture and lubrication of constant velocity joints has provided joints with relatively long life, it is not uncommon for the driveaxle to fail long before the remainder of the vehicle has deteriorated to an extent which would render it unserviceable. Thus, there is a significant market for replacement driveaxles. Due to the significant cost of some of the driveaxle components, and the fact that only a limited number of the components are likely to fail, an industry has grown up around rebuilding driveaxles. Some rebuilding shops use relatively sophisticated equipment such as the constant velocity joint tester described and claimed in commonly owned Philpott et al U.S. application Ser. No. 08/468,138, filed Jun. 6, 1995, allowed (Attorney Docket 62877). Others rely on visual inspection. But in any event, ultimately judgments are made as to which parts should be replaced, which can be repaired or re-machined and which can be reused without repair. In the end, the CV joint is re-assembled, hopefully with non-defective parts, and two joints are assembled to an axle to provide a rebuilt driveaxle. It would be useful to test such an axle before selling it or installing it in an automobile. However, due to the characteristics of the driveaxle, particularly the intermittent manner in which they exhibit faults, such a test has not been readily available.
In the CV joint rebuilding industry where many CV joints are rebuilt daily, any axle testing device must be simple to use and reasonably foolproof. Because many different kinds of driveaxles of different sizes and shaft configurations are used on vehicles by different manufacturers, it is necessary that the embodiment of the testing apparatus be rapidly adapted for testing different axles.
Even in the setting of a regular automobile repair shop not specialized in rebuilding CV joints, a reliable and inexpensive axle testing apparatus is valuable for diagnostic purposes. When the drive train of an automobile develops vibration and noise, the cause could lie in any part of the drive train. With a convenient driveaxle testing system, a mechanic can find out quickly whether the axle is the cause of the drive train problem. Thus an axle testing system will be useful for locating drive train problems and for avoiding the undesirable mistake of replacing a good axle without fixing the problem.